1. Field of the Invention
The invention relates to an exhaust gas purification catalyst including an iron oxide-zirconia-based composite oxide.
2. Description of Related Art
Exhaust gases discharged from the internal combustion engines of motor vehicles and the like contain harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). In exhaust gas purification catalysts (so-called ternary catalysts) for decomposing such harmful gases, ceria-zirconia composite oxides and the like having oxygen storage capacity (OSC) are used as promoters. Substances (oxygen storage materials) having OSC absorb and release oxygen to thereby control the air-fuel ratio (A/F) in the microscopic spaces, and have the effect of inhibiting the degree of cleaning from decreasing with fluctuations in exhaust gas composition.
Cerium, which is often contained in composite oxides having OSC, is not only expensive but also becoming less stably available due to the worsening of the recent procuration situation. Accordingly, exhaust gas purification catalysts including iron as an oxygen storage material are being investigated and developed for the purpose of reducing the use amount of cerium.
For example, Japanese Patent Application Publication No. 2007-136339 (JP 2007-136339 A) discloses particles for catalytic use which function to storage and release oxygen and which each include a core including the oxide of Fe having a valence of 3 as a main component and a layer formed on the peripheral surface of the core, the layer being constituted of an iron compound that is a solid solution of a divalent element, other than iron, in the oxide of iron having a valence of 3. Japanese Patent Application Publication No. 2009-285619 (JP 2009-285619 A) discloses a catalyst for exhaust gas purification which is characterized in that a large number of iron oxide particles are dispersedly contained in the catalyst layer, at least some of the iron oxide particles are fine iron oxide particles having a particle diameter of 300 nm or smaller, the fine iron oxide particles being in contact with cerium-containing oxide particles having oxygen storage/release capacity, and the areal proportion of the fine iron oxide particles to the total area of the iron oxide particles being 30% or higher.
Fe2O3 has OSC due to the oxidation/reduction. Meanwhile, iron undesirably reacts with other catalyst materials such as, for example, ceria-zirconia solid solutions and alumina. For that reason, persons skilled in the art have recognized that oxygen storage materials utilizing Fe2O3 are prone to decrease in oxygen storagecapacity and that catalysts including such oxygen storage materials are prone to decrease in cleaning performance and thereby have poor durability. The inventors addressed this problem and previously succeeded in greatly improving the durability by adding ZrO2, La2O3, etc. to Fe2O3 to obtain an iron oxide-zirconia-based composite oxide. However, the durability thereof is still insufficient.